1. Field of the Invention
The invention relates to apparatus and methods related to optical spectroscopy and to optical sensors.
2. Description of the Related Art
Optical spectrometers can be generally divided into dispersive or diffractive spectrometers and Fourier transform spectrometers. Dispersive (prism) spectrometers or diffractive (grating) spectrometers break down the incident light beam into its spectral components by the wavelength dependence of an angle of deflection or of an angle of reflection. The different spectral components are thereby spatially separated and the spectral component to be determined can be selected (monochromator). The detection of a spectrum then takes place with the help of moving parts in that the different spectral components are selected and measured in succession.
Monchromators are most common with a Czerny-Turner beam path, i.e. with a rotatable planar grating (diffraction grating in reflection) between an entry slit and an exit slit and collimator mirrors or collector mirrors independent of one another. The collimator and collector effect an imaging of the entry slit in the plane of the exit slit. The diffraction grating is located in the Fourier transform plane of this imaging system.
The development of spatially resolving detectors (CCD, diode array) now permits the simultaneous measurement of all spectral components in that a separate element of the detector is provided for each spectral component. Such an arrangement (polychromator) manages without any moving parts and utilizes the available incident light substantially more efficiently.
Fourier transform spectrometers are based on an interferometer in which the difference of the optical path lengths of the partial beams brought to interference can be set with high precision. The spectrum can be determined by Fourier transformation from a measurement of the interference signal via a suitable range of path length differences.
Instruments are normally set up in the manner of a Michelson interferometer or of a Twyman-Green interferometer. The mechanical components for the setting of the optical path lengths by moveable mirrors or tiltable mirror pairs and the required collimator for the generation of planar wavefronts are above all technically demanding here.
A further variant of spectrometers uses static interference patterns generated by light beams which are brought to interference at a specific angle, e.g. Fizeau interferometers. The spectrum can be calculated by counting the interference stripes or via a determination of the spatial frequencies of the interference pattern with the help of a numerical Fourier transformation.
The fact is disadvantageous for these interferometric spectrometers (both for Michelson/Twyman Green interferometers with variable wavelengths and for static interferometers with spatial interference patterns) that the relative spectral resolution is determined directly by the number of the line pairs (Fizeau stripes) measured in the interference patterns. If N line pairs are counted for a specific wavelength .lambda., the spectral resolution lies in the order of magnitude of .lambda./N.
A more recent variant of Fourier transform spectrometers (“spatial heterodyne spectrometers”) uses dispersive or diffractive optical elements (diffraction gratings) in order to change the angle between two collimated partial beams of a static interferometer as a function of the wavelength and so to increase the spectral resolution.
The superposition of planar wavefronts is necessarily required here to obtain Fizeau interferograms (Fizeau stripes) which can be broken down into their spectral components by a numerical Fourier transformation after the measurement.
Such arrangements are furthermore based on the translation invariance of the optical Fourier transformation. The incident light is first collimated by a collimator. The collimated beam (planar wavefronts) is divided (amplitude division) and guided over spectrally dispersive or diffractive elements, e.g. over a diffraction grating. The spectrally dispersive optical element lies in the Fourier plane of the collimator in this process. The partial beams, which are superposed again, are then imaged through a collector and a further Fourier transform lens such that a spatially resolving detector again comes to rest in a Fourier transform plane of the entry aperture.
Such arrangements—like Fourier transform spectrometers or conventional monochromators—are therefore dependent on imaging optical systems of high quality. Relatively large focal lengths of the optical systems are in particular required.
The possible performance capability of dispersive or diffractive spectrometers depends on specific parameters, in particular on the dimensions of the entry slit or the exit slit, on the focal length and aperture of the imaging elements and on the properties of the dispersive or diffractive element itself. Modem instruments almost reach these physically set limits.
The possible performance capability of Fourier transform spectrometers is correspondingly determined by specific parameters, and here in particular by the range and the increment for the variation of the optical path lengths. The performance capability of Fourier transform spectrometers greatly surpasses the possibility of dispersive or diffractive spectrometers.
Fourier transform spectrometers can also almost reach the physical limits of their performance capability, but the technical effort is very high in many cases. Since Fourier transform spectrometers are based on an interferometer, all optical components, and in particular also the moving parts, must be produced and positioned with a precision of fractions of the wavelengths to be measured.
Spatially heterodyne spectrometers are technically less complex, but likewise need both imaging optical components of high quality and dispersive or diffractive optical components of high quality.
The spectral resolution d.lambda. at a wavelength .lambda. of all named spectrometers is directly related to a corresponding coherence length 1=.lambda.sup.2/d.lambda.
To achieve a specific spectral resolution, the spectrometric arrangement must generate defined differences of the optical path lengths of at least the range 1.
The necessity of a collimation of the incident light is thus common to all named spectrometers. The collimator is an imaging optical element of a specific focal length f, e.g. a concave mirror or a lens. The entry aperture of the spectrometer is located at the focal point of the collimator.
The spectrometers now explicitly utilize the special properties of the optical Fourier transformation, in particular the translation invariance of the Fourier transformation, i.e. the transformation of a translation in the focal plane to a change of the direction of propagation in the Fourier plane of the collimator.
Monochromators (“4f system” with focal length f: entry slit-f-collimator-f-diffraction grating-f-collector-f-exit slit) influence the propagation direction of the light in the Fourier plane of the imaging system by means of a diffraction grating and thus generate the desired spectral dispersion without essentially disturbing the imaging of the entry slit onto the exit slit or detector (with 1 being defined by the geometry of the grating in the beam path, f>>1). The collimator carries out an optical Fourier transformation, the collector takes over the optical retransformation and thus effects the optical imaging of the entry slit into the plane of the exit slit or of the detector.
Fourier transform spectrometers (2f system) necessarily require the collimator (as a rule with f substantially larger than 1) to maintain the interference despite optical paths of different lengths, i.e. to superpose the wavefronts suitably at the detector. The translation invariance of the Fourier transformation is in particular utilized here.
With a Fourier transform spectrometer, the numerical Fourier transformation replaces the optical retransformation used with the monochromator.
Fourier transform spectrometers with dispersive elements, which evaluate a spatial interference pattern (spatially heterodyne spectrometers) explicitly require the collimator in the context of an optical Fourier transformation, on the one hand to avoid a blurring of the interference patterns despite a finitely large entry opening (translation invariance), on the other hand to establish the defined and unambiguous relationship between the optical spectrum and corresponding spatial frequencies in the resulting pattern which forms the basis for the numerical retransformation.
These spectrometers moreover require an additional optical imaging system (“6f system”: entry slit-f-collimator-f-interferometer with diffraction grating-f-collector-f-exit diaphragm-f-imaging element-f-detector plane).
Since both interferometric arrangements and systems imaging at high resolution have to be realized through high-quality optical systems, with large focal lengths as required, and since a minimum size of the components or of the path lengths is fixedly predetermined by the aforesaid value 1—in dependence on the respective exact arrangement, the technical effort increases quickly as the demands on the spectral resolution grow. A characterizing parameter here is the so-called spectral aperture broadening which occurs despite collimation.